Radioactive 129I is one of the longer-lived fission products (1.6×107 years) resulting from the generation of energy from nuclear fuels, and it is also one that is associated with considerable public concern by virtue of the mechanism whereby it may become concentrated in the human body where it can potentially have adverse health effects. Until recently in France, 129I was discharged to the ocean for isotope dilution with the natural iodine in seawater.
With the growth of research on advanced fuel cycles in the United States and abroad, there is a strong interest in the separation and waste form development for all radioisotopes that are present in spent nuclear fuel once the components that can be incorporated into new fuel rods have been removed. This includes the initial trapping of gaseous iodine radioisotopes, and their incorporation into waste forms. During spent fuel reprocessing, the gaseous forms of radio-iodine (principally I2, CH3I, HI, and HIO) must be captured in a form that is suitable for long-term storage. Whether wastes are slated for above ground storage, or underground burial, a serious need is that the radionuclides (e.g., 129I) exist in highly insoluble chemical forms that will not be readily dissolved should water gain access to the site.
A second major consideration is that the wastes not exist as powders, since an accident during storage or handling could produce a cloud of radioactive dust with the potential for causing widespread contamination.
Nuclear fuel reprocessing is a technology that has been under development for more than half a century. During normal reprocessing activities, as the spent fuel is dissolved from the nuclear fuel rods, most of the radio-iodine is liberated and leaves as elemental iodine vapor. An international consensus has developed that incorporating radioisotopes into borosilicate glass waste forms is a convenient and acceptable (though not necessarily optimal) technology. Iodine, however, remains a notable exception, because conventional glass waste forms do not retain the iodine due to the high temperature necessary to melt the glass.
At this time, the leading technology for capturing radio-iodine from the reprocessing off-gases is sorption onto a silver-loaded zeolite matrix (where the iodine reacts with silver to form silver iodide, AgI). Recent studies at Sandia indicate that the iodine is sequestered in the form of sub-micron sized silver iodide (AgI) crystals on the internal and external surfaces of zeolite particles. One of our important research findings was that if the silver is loaded to the bulk surface (as opposed to ion exchanged into the zeolite pore), much of the iodine will be trapped on the bulk surface of the zeolite crystals, with only some of it in the channels and pores of zeolite crystal. Because of surface entrapment, mild heating causes easy release of the iodine as iodine gas. Additionally, zeolites are crushable metal oxides, and can easily form powders and dust if not protected from mechanical damage.
A different approach to solving this problem is to heat the silver-loaded zeolite matrix at a temperature sufficiently high (500°-700° C.), with or without pressure, to collapse the porous framework and create a densified/sintered ceramic that retains the iodine as AgI. However, the sintering temperature cannot be so high as to cause sublimation of the AgI (˜600° C.), causing subsequent release of gaseous iodine. Unfortunately, in recent tests, commercially available silver-loaded zeolites were sintered, but did not produce the expected sequestering result because too much iodine was released during processing (likely due to the surface entrapment effect).
Either as produced by reaction with a Ag-zeolite or through direct reaction with metallic silver powder, AgI is a common host for 129I. AgI has a very low solubility in water as compared to other iodides (3×10−6 g/L or 1.3×10−8 mol/L at 20° C.), but has a relatively high vapor pressure at moderate temperatures. It undergoes a β to α phase change at 147° C., and it melts at 558° C. It has a vapor pressure of 10 mTorr at 600° C., which limits the thermal processing temperature. Thermal gravimetric analysis confirms that AgI begins to volatilize appreciably above 600° C. Known borosilicate glass-based waste forms are produced by melting the glass at high temperatures, >1000° C. Recent work has explored using low temperature (550° C.) sintering glass to encapsulate AgI or AgI-zeolite. However, this approach still requires thermal processing and is not suitable for use with even more temperature sensitive iodine absorbers such as metal-organic framework materials (MOFs) that can trap much higher levels iodine but typically began to decompose and/or release iodine at temperatures as low as 150° C.
Hence, a need exists for a highly stable binder or encapsulant material that securely sequesters particles of temperature sensitive waste, such as AgI, AgI-zeolite or iodine containing MOFs; and that has good mechanical strength, durability, low iodine outgassing, and low rates of leaching in groundwater.
Dense and durable waste forms for nuclear waste capable of room temperature fabrication using densifiable powder material, such as metal powder, that forms the matrix that encapsulates the radioactive components of the waste that do not suffer from one or more of the above drawbacks would be desirable in the art.